This invention relates to an apparatus for generating an image, and more particularly to an apparatus for generating an input image for a holographic waveguide display.
The invention addresses the problem of providing uniform output illumination in holographic waveguide displays. U.S. patent application Ser. No. 13/844,456 entitled TRANSPARENT WAVEGUIDE DISPLAY, PCT Application No.: GB2012/000677 entitled WEARABLE DATA DISPLAY, U.S. patent application Ser. No. 13/317,468 entitled COMPACT EDGE ILLUMINATED EYEGLASS DISPLAY, U.S. patent application Ser. No. 13/869,866 entitled HOLOGRAPHIC WIDE ANGLE DISPLAY, and U.S. patent application Ser. No. 13/844,456 entitled TRANSPARENT WAVEGUIDE DISPLAY all of which are incorporated herein by reference in their entireties.
The invention addresses a particular category of holographic waveguides based on Switchable Bragg Gratings (SBGs). SBGs are fabricated by first placing a thin film of a mixture of photopolymerizable monomers and liquid crystal material between parallel glass plates. One or both glass plates support electrodes, typically transparent indium tin oxide films, for applying an electric field across the film. A volume phase grating is then recorded by illuminating the liquid material (often referred to as the syrup) with two mutually coherent laser beams, which interfere to form a slanted fringe grating structure. During the recording process, the monomers polymerize and the mixture undergoes a phase separation, creating regions densely populated by liquid crystal micro-droplets, interspersed with regions of clear polymer. The alternating liquid crystal-rich and liquid crystal-depleted regions form the fringe planes of the grating. The resulting volume phase grating can exhibit very high diffraction efficiency, which may be controlled by the magnitude of the electric field applied across the film. When an electric field is applied to the grating via transparent electrodes, the natural orientation of the LC droplets is changed causing the refractive index modulation of the fringes to reduce and the hologram diffraction efficiency to drop to very low levels. Note that the diffraction efficiency of the device can be adjusted, by means of the applied voltage, over a continuous range. The device exhibits near 100% efficiency with no voltage applied and essentially zero efficiency with a sufficiently high voltage applied. In certain types of HPDLC devices magnetic fields may be used to control the LC orientation. In certain types of HPDLC phase separation of the LC material from the polymer may be accomplished to such a degree that no discernible droplet structure results. SBGs may be used to provide transmission or reflection gratings for free space applications. In waveguide applications the parallel glass plates used to form the HPDLC cell provide a total internal reflection (TIR) light guiding structure. Light is “coupled” out of the SBG when the switchable grating diffracts the light at an angle beyond the TIR condition. Typically, the HPDLC used in SBGs comprise liquid crystal (LC), monomers, photoinitiator dyes, and coinitiators. The mixture frequently includes a surfactant. The patent and scientific literature contains many examples of material systems and processes that may be used to fabricate SBGs. Two fundamental patents are: U.S. Pat. No. 5,942,157 by Sutherland, and U.S. Pat. No. 5,751,452 by Tanaka et al. Both filings describe monomer and liquid crystal material combinations suitable for fabricating SBG devices. One of the known attributes of transmission SBGs is that the LC molecules tend to align normal to the grating fringe planes. The effect of the LC molecule alignment is that transmission SBGs efficiently diffract P polarized light (ie light with the polarization vector in the plane of incidence) but have lower diffraction efficiency for S polarized light (ie light with the polarization vector normal to the plane of incidence.
Waveguides offer many features that are attractive in HMDs and HUDs. They are thin and transparent. Wide fields of views can be obtained by recording multiple holographs and tiling the field of view regions formed by each hologram. A key feature of these waveguides is that they provide pupil expansion in two orthogonal directions. The pupil expansion in a given direction is achieved by diffracting equal amounts of light out of output grating toward the eye box at each beam grating interaction. Uniformity of output is achieved by designing the output grating to have diffraction efficiency varying from a low value near the input end of the waveguide to a high value at the furthest extremity of the output grating. The inventors refer to grating having such properties as lossy gratings. The diffraction efficiency profile along the waveguide may be controlled by varying one or both of the grating refractive index modulation and the grating thickness. According to the theory of Bragg gratings higher index modulations give higher peak efficiency and narrow diffraction efficiency angular bandwidths. Reducing the thickness of the grating leads to a decrease in the diffraction efficiency and a broadening of the diffraction efficiency angular bandwidth. The input image data is provided by a microdisplay external to the waveguide. The microdisplay which is usually a reflective array must be illuminated via a beam splitter. The reflected image light is collimated such that each pixel of the image provides a parallel beam in a unique direction. Finally, the image light must be coupled efficiently into the waveguide so that the image content can by transferred to the waveguide components used for orthogonal pupil expansion. The image light from the microdisplay is normally coupled into the waveguide via an input grating. Alternatively a prism may be used.
A major design challenge is coupling the image content into the waveguide efficiently and in such a way the waveguide image is free from chromatic dispersion and brightness non uniformity. To overcome chromatic dispersion and to achieve the best possible collimation it is desirable to use lasers. However, lasers and other narrow band sources such as LEDs suffer from the problem of pupil banding artifacts which manifest themselves as output illumination non uniformity. Banding artifacts are formed when the collimated pupil is replicated (expanded) in a TIR waveguide. In very basic terms the light beams diffracted out of the waveguide each time the beam interacts with the grating have gaps or overlaps. This leads to an illumination ripple. The degree of ripple is a function of field angle, waveguide thickness, and aperture thickness. The inventors have found by experiment and simulation that the effect of banding can be smoothed by dispersion with broadband sources such as LEDs. The effects are therefore most noticed in narrowband (e.g. laser) illumination sources.
There is a requirement for an image generator for illuminating a microdisplay, collimating the reflected image light from the microdisplay and efficiently coupling it into a thin holographic waveguide with high efficiency, with low chromatic dispersion and with high illumination uniformity.
There is a further requirement for a waveguide display comprising an image generator for illuminating a microdisplay, collimating the reflected image light from the microdisplay and efficiently coupling it into a thin holographic waveguide with high efficiency, with low chromatic dispersion and with high illumination uniformity, and holographic waveguides for providing pupil expansion in orthogonal directions.